Method and apparatus for weed control with hot foam

ABSTRACT

A method of controlling vegetation including the steps of: (a) preparing a relatively ‘wet’ foam having a temperature between 75° C. and 100° C. and consisting volumetrically of 60% to 75% aqueous solution of biodegradable foaming agent and 40% to 25% air, and (b) directing a stream of the foam against the surface to be treated. An apparatus for applying the foam to vegetation including a supply tank for holding an acqueous foamable solution, a pump for supplying the solution to a boiler for heating, a blower for supplying air, hoses for carrying air and hot solution respectively to a lance incorporating a mixing and expansion chamber.

TECHNICAL FIELD

The present invention relates generally to the thermal control ofvegetation, particularly weeds, and more specifically to a method andapparatus for the control of weeds using hot foam.

BACKGROUND ART

Thermal methods have been proposed for weed control wherein a variety ofheated materials are applied to the weeds, including hot water, steam,hot air, flame and hot foam. These methods overcome some of thedisadvantages of herbicides, such as spray drift and soil/groundwaterpollution. They also avoid the physical effort required for manual weedremoval.

New Zealand patent no. 240568 describes the use of hot water appliedunder a shroud. Compared to herbicides, the method is fast-acting,effective over a range of vegetation and weather conditions, and beingnon-toxic its use is less restricted e.g. personal protective clothingor certification is not required for applicators. However, the longshroud (to provide a thermal screen for temporary insulation fromambient temperature and wind) restricts the use and speed of coverage ofhot water systems.

An alternative is described in U.S. Pat. No. 5,433,758 wherein a sprayof hot water is applied to the weed, followed by a cold foam blanket toprovide the temporary thermal screen. This method, however, requiresseparate apparatus for the production and distribution of the hot waterand foam. Also, some cooling results from the application of a cold foamblanket which reduces the effectiveness of this method.

U.S. Pat. No. 5,575,111 describes a method where heated compressed airis mixed with a stream of hot water and surfactant to form a hot foamfor application to the weeds. In this method heated air is used to meltthe plants waxy cuticular coating while the foam acts to provide thetemporary thermal screen. The foam used is relatively dry, being from 80to 95% hot air by volume. It has been found however, that this method isnot particularly effective at killing weeds. It appears that severalfactors may influence the lack of effectiveness of this method. Althoughthe high temperature air disrupts the cuticle, it has been found thatweeds treated in this manner grow back more quickly than those treatedwith hot water systems. Relatively dry foams have a low heat capacityand have a reduced ability to penetrate below the outermost surface ofthe plant to produce deeper more long lasting damage, particularly tothe harder surfaces such as the stems of the target plants. Due to thestiffness of relatively dry foams they are unable to flow to obtain goodintimate contact with the plants, thereby protecting, for example, theunderside of foliage from contact with the foam. The size of bubblesformed in the foam is also an important factor, as large bubbles alsoreduce the heat flux through the foliage and stems with which they arein contact.

It is an object of the present invention to provide a system of thermalweed control which overcomes the drawbacks of hot water methods whileretaining its advantages, but which also addresses the foregoingproblems of foam methods.

Further aspects and advantages of the present invention will becomeapparent from the ensuing description which is given by way of exampleonly.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided amethod of controlling vegetation, including the steps:

-   -   preparing a foam having a temperature of between 75° C. and        100° C. and consisting volumetrically of 60% to 75% aqueous        solution of biodegradable foaming agent and 40% to 25% air, and    -   directing a stream of the foam against the surface to be        treated.

Although the method is primarily intended for the control of vegetation,particularly weeds, this should not be seen as limiting as it is alsoapplicable to soil, for example, and for killing or controlling fungus,bacteria, virus and insects etc.

Preferably the foam is prepared by heating the aqueous solution andmixing a stream of air with a stream of aqueous solution to produce thestream of foam which is discharged at low pressure.

The foam discharge pressure may range from 5 PSI to 15 PSI, but ispreferably 10 PSI (0.7 bar).

Preferably the water and foaming agent are mixed in the correctproportions in a reservoir. Optionally, the foaming agent may be meteredinto a flow of water, before or after the flow is heated.

Preferably the aqueous solution is alkyl polygclycoside (APG) mixed withwater at a dilution rate of approximately 0.4%. Optionally otherbiodegradable foaming agents maybe employed.

Preferably the solution is pumped through a boiler into a mixing device,where it is mixed with the air to produce foam which then passes throughan elongate delivery passage before being discharged through a divergentshroud.

The solution delivery rate for commercial applications may be from 10litres per minute to 500 litres/minute, or more. In the preferredembodiment, the foamable solution is supplied at a rate of approximately10 litres per minute and at a pressure in the range of 5-15 PSI and mostpreferably 10 PSI.

The method produces a wet foam which consists volumetrically of 60% to75% aqueous solution. Generally, this volumetric ratio should notnormally exceed 85% with 60% being about optimum. Optionally, the streamof low pressure air may be heated prior to mixing with the solution.

According to another aspect of the present invention there is provided amethod of controlling vegetation, including the steps:

-   -   a) providing a flow of hot aqueous solution of biodegradable        foaming agent;    -   b) passing at least part of said flow through a channel to        facilitate low pressure that creates a venturi to draw in a        stream of ambient air;    -   c) mixing the stream of air with the flow of hot aqueous        solution to produce a stream of foam having a temperature of        between 75° C. and 100° C. and consisting volumetrically of 60%        to 75% aqueous solution of biodegradable foaming agent and 40%        to 25% air, and    -   d) discharging the stream of foam against the surface to be        treated.

Optionally, two or more channels may be provided, each drawing in arespective stream of ambient air to foam the solution. Preferably eachchannel provides a free jet of solution.

Preferably the solution is pumped through a boiler, the full streamflowing through one channel to facilitate low pressure that creates aventuri to draw in the stream of ambient air into a mixing chamber,where it is mixed with the solution to produce foam that then passesthrough an elongate delivery passage before being discharged through adivergent shroud.

According to another aspect of the present invention there is provided aprovided an apparatus for performing the method of controllingvegetation substantially as described above, the apparatus including:

-   -   means for supplying a stream of hot aqueous solution;    -   means for supplying a stream of air;    -   a mixing chamber where the streams of air and solution are mixed        to form foam, and    -   an elongate delivery passage connected between the mixing        chamber and a divergent outlet shroud.

Preferably the solution is directed through a conduit to enter themixing chamber in a jet-like flow substantially perpendicular to the airflow.

Preferably a foam control mesh is provided in the foam flow path. Themesh may be positioned in the shroud, near the outlet thereof. The meshpreferably has generally 4.76 mm holes at 6.35 mm triangular pitch, 51%open area.

Preferably the internal cross-sectional area of the delivery passage isapproximately 60% greater than that of the air delivery conduit andapproximately ten times that of the solution delivery conduit.

Preferably the foam is applied from a mobile means, most preferably aself-propelled vehicle e.g. a truck or tractor.

In the apparatus for performing the method of controlling vegetationsubstantially as described above, the means for supplying a stream ofair includes channel means which facilitate low pressure that creates aventuri to draw in a stream of ambient air.

In an embodiment for controlling vegetation by the application of hotfoam through a hand lance, the venturi created by flow through thechannel is preferably mounted on the hand lance. Preferably theapparatus includes a hand-operated valve for controlling the flow of airinto the mixing chamber.

As well as reducing equipment cost and energy usage, it has been foundthat using a venturi as described reduces the drop in temperaturebetween the solution and the foam, producing a higher temperature foamfor more effective treatment, whether using a hand lance, a vehiclemounted system or otherwise. It is possible when using a hand lance toconsiderably increase the length of hose that can be employed, thusproviding the operator with additional versatility.

This apparatus and method is particularly useful to enable betterapplication of heat in places where application is difficult, forexample dense swards and vegetation substantially above 200 mm inheight. The application is less confined by surface conditions andobstacles since the foam adheres to vertical surfaces and does not runoff like liquids. The residence time of hot foam on target areas isgreater than that of hot water. The evaporation rate of wet foams isless, resulting in longer contact with the target. Also, foams providevisual evidence of coverage of the application area. Faster and morelong-lasting control of weeds is provided, compared to the known hot airfoam method.

With the application of hot foam instead of pure hot water a thermalscreen is superfluous because the foam performs this function. Unlikepure hot water the hot foam can be blown over a certain distance withoutconsiderable heat loss.

Compared with other thermal methods it is distinguished by considerablyhigher working speed and lower costs. An area may be covered morequickly with lower water consumption and energy input per square meterthan is possible with hot water systems.

Besides weed control in horticulture and agriculture, other applicationareas include the control of vegetation around trees, at the edge ofconcrete or paving, on roadside berms, around buildings and otherobstructions, along railway lines, and other areas where the overgrowthof vegetation and in particular weeds is to be controlled.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from thefollowing description which is given by way of example only and withreference to the accompanying drawings in which:

FIG. 1 is a schematic view of preferred embodiment of a handhelddelivery system for performing a method of the present invention;

FIG. 2 is a side elevation of the hand lance of FIG. 1;

FIGS. 3 a and 3 b are plan and side elevations respectively of theshroud of the hand lance of FIG. 1;

FIG. 4 is a first alternative boom mounted system for performing amethod of the present invention, and

FIG. 5 is a second alternative boom mounted system for performing themethod of the present invention;

FIG. 6 is a side elevation of the hand lance of a delivery system forperforming a method of the present invention;

FIG. 7 is a section of the hand lance of FIG. 6, and

FIGS. 8 a and 8 b are plan and side elevations respectively of theliquid jet of the hand lance of FIG. 7.

BEST MODES FOR CARRYING OUT THE INVENTION

As illustrated in the drawings, the apparatus is mounted to a vehicle(not shown), preferably a truck and includes an aqueous foamablesolution supply tank 1, a boiler 2 for heating the solution from thetank 1 and a blower 3 for supplying air to foam the solution. The boiler2 is positioned adjacent to the tank 1, and solution is supplied to theboiler 2 by a pump 4 powered by electricity from a generator 5. In onepresently preferred embodiment, the boiler 2 has a burner (not shown)which burns liquid fuel, and the pump 4 is a single-stage centrifugalpump. Alternatively, the pump may be driven directly by a small petrolengine.

Referring to FIGS. 1 and 2, a hand delivery system is illustratedwherein air from the blower 3 and hot solution from the boiler 2 aresupplied by hoses 6 and 7 respectively to a hand lance 8 at the distalend thereof. The lance 8 has a trigger-operated valve 9 to control theflow of the hot solution to a mixing and expansion chamber 10 where itis introduced into the air stream to produce foam which then passes intoan elongate delivery tube 11, before being discharged from a divergentshroud 12 for directing the pressurized foam against the surface to betreated. Optionally, a wheel 13 may be fitted to by means of a bracket14 to support the lance 8.

FIGS. 3 a and 3 b illustrate in more detail the shroud 12, whichdiverges from a circular section at the delivery tube end to a generallyrectangular outlet 15, having an included angle of approximately 60degrees. Located in a medial section of the shroud 12, a foam controlmesh 16 is provided which is arranged approximately perpendicular to theflow. The mesh 16 is preferably bent into a 90° V about an axisperpendicular to the flow direction.

Foam producing apparatus as known heretofore, are primarily intending toefficiently produce foam having a relatively low moisture content forapplications such as fire fighting, herbicide application or forinsulating purposes (e.g. U.S. Pat. No. 5,575,111). For high efficiencyin these applications it is important to have a high expansionratio—that is to produce a high volume of foam in relation to thequantity of liquid solution and compressed gas employed.

The system of the present invention, however, results in a relativelylow expansion ratio producing wet foam with reduced cooling and a foamtemperature of approximately 92° C. or higher in the preferredembodiment.

While the apparatus shown is illustrative of the requirements forperforming the method of the present invention, it will be appreciatedthat many variations of the apparatus may be employed to perform themethod. For example, if desired, instead of being mixed with water inthe tank 1, the foaming agent can be introduced into hot water at theoutput of the boiler. The foaming agent may stored in a separate tank(not shown) and introduced into the hot water stream from the outlet ofthe boiler, with a flow control valve (not shown) for controlling theamount of agent which is introduced, alternatively a metering pump (notshown) may supply the foaming agent.

To utilize the apparatus above described for the purpose intended, thefoamable solution preferably comprises alkyl polyglycoside (APG) mixedat a 0.4% dilution rate with water. APG is able to withstandtemperatures in excess of 100° C. without adverse effects and produces afoam which is stable for between 5 to 20 minutes under most ambientconditions. Moreover, the foam produced breaks down without drifting offthe target plant and APG is a naturally occurring product whichbiodegrades both aerobically and anaerobically.

The output of the boiler 2 consists of pressurised solution at atemperature of close to 100° C. Actuation of the trigger 9 directs thesolution into the stream of air from the blower 3. As the air mixes withthe liquid solution it creates bubbles forming foam which builds up andis forced from the chamber 10 toward the outlet by the air pressure. Airis supplied at a rate of approximately 0.7 m³ per minute, with solutionsupplied at approximately 10 litres per minute—the apparatus producing afoam which is approximately 60% solution by volume and at a temperatureof 92° C., or more.

Factors which have been found to affect the correct foam productioninclude the dimensions of the shroud 12 and the delivery tube 11. A 32mm internal diameter delivery tube of approximately 400 mm lengthdiverging to a shroud opening of approximately 200 mm×15 mm has beenfound to give good results. The internal cross-sectional area of thedelivery tube is approximately ten times that of the tube supplying thesolution.

It is thought that the relative lack of foam flow constriction resultingfrom this arrangement is important in producing the foam, which ispressurised only to approximately 10 PSI (0.7 bar). Higher pressures andair flows have been found to disperse the foam from the target andresult in excessive cooling. The mesh 16 provided in the shroud 12controls bubble size and foam formation and a mesh 16 having generally4.76 mm holes at 6.35 mm triangular pitch, and 51% open area has beenfound satisfactory.

The cross-sectional area of the chamber 10 is approximately 60% largerthan that of the air hose 6. This, accordingly, does not permit a largeexpansion and a volume of wet, high density foam is continuouslyproduced in response to the continuous agitation of the foamablesolution by the air flow.

In preferred embodiments the generator 5, tank 1, pump 4, boiler 2 andblower 3 are sized to provide foamable solution and air to two lances 8,which are operable separately or together. The system is capable oftreating a nominal 1000 m2 per hour, at an approximate treatment widthof 200 mm to 300 mm from each lance.

FIGS. 4 and 5 illustrate schematically systems for higher applicationrates, where for example, the outlet shroud 112 is mounted on a boom(not shown) fixed to a vehicle. Like numbering is used to refer to theelements common with the system of FIG. 1. Instead of a manuallyactuated trigger valve, a remotely actuable valve 20 (e.g. a solenoidvalve) operated by the vehicle's driver controls the supply of thefoamable solution. The foam may be generated either adjacent to theboiler outlet with a large diameter delivery hose 21 supplying it to theshroud 21 (as per FIG. 4) or the foam may be generated adjacent to theshroud 112, requiring elongation of the separate air and solution lines(as per FIG. 5).

As used in FIGS. 6, 7, 8 a and 8 b, like numbers refer to componentscommon with the lance described with reference to FIGS. 2 and 3.Flowrates, pressures, temperatures etc are also as per the descriptionof the preferred embodiment of the invention described with reference toFIGS. 2 and 3.

As illustrated in FIG. 7, a hand lance 80 for performing the method ofthe present invention is connected to a supply of hot aqueous solutionof biodegradable foaming agent through supply hose 7. The lance 80 has atrigger-operated valve 9 to control the flow of the hot solution to amixing and expansion chamber 100 into which ambient air is introducedthrough opposing openings 31 to produce foam which then passes into anelongate delivery tube 11, before being discharged from a divergentshroud 12 for directing the pressurized foam against the surface to betreated.

FIGS. 7, 8 a and 8 b illustrate the construction of the chamber 100 inmore detail. The chamber 100 comprises a tubular section 33 joined to aconical section 34 tapered out to the larger diameter of the deliverytube 11. An opening at the end of the tubular section 33 receives anozzle 32 having a central outlet 35 which produces a single jet ofsolution directed longitudinally through the chamber 100.

As will be understood the flow through the nozzle 32 facilitates lowpressure that creates a venturi to draw in a stream of ambient airthrough the openings 31. This system results in a relatively lowexpansion ratio producing wet foam with reduced cooling and a foamtemperature of approximately 92° C. or higher in the preferredembodiment.

With the operating parameters described in application no. 514497,satisfactory results have been obtained employing a jet having a 4.5 mmdiameter directing flow into a tubular section 33 having a 19 mm bore,with both the tubular section 33 and conical section 34 havingsubstantially the same lengths. The apparatus produces a regular foamstructure and more even temperature spread across the shroud 12.

It will be appreciated however, that this method of introducing the airfor foaming the solution can be employed in a number of differentapplications, and is also not limited to applications employing a handlance.

In an alternative embodiment (not shown) a sliding cover is provided forengagement with the tubular section 33 to form a valve that may becontrolled for regulating the flow of air through the openings 31. Thisvalve may be closed allowing the lance to be used for purposes otherthan foam production e.g. spraying. Varying the opening of the valveallows the composition and texture of the foam to be adjusted, as may bepreferable for treating different types of vegetation, for example.

On the hand lance, the valve may be directly controlled by the operator.The valve also provides a method of protecting the operator. It can beclosed when a sensor measures a temperature in excess of 100° C. toprevent the escape of steam which (because of the position of the airinlet on the lance) may blow back on the operator.

To achieve higher foam production rates, satisfactory results have beenobtained by maintaining the ratio (1:70) between the solution and flowrates air e.g. 40 litres per minute solution flow rate requires 2.8 m³per minute of air supply.

In preferred embodiments for large scale commercial applications, it isanticipated that spray booms of up to 10 metres in width could be used,with proportionate increases in the flow rates to provide the specifiedfoam temperature and composition.

Trials have been conducted to examine the effect of a number ofvariables associated with the present apparatus and method of producinghot foam. These trials determined the following important features.

-   -   Foam texture and temperature are critical for the best results.        The texture or viscosity level, which is influenced by the        wetness and degree of aeration, is very important. A cold        foamable solution mixed in a tank and then delivered into the        heating chamber produced the most accurately repeatable        performance. It is possible that when using an injection or        venturi delivery system for introducing the foaming agent,        accuracy of mix became variable due to foam agent building up in        delivery lines and residues remaining in the delivery chamber. A        temperature range of 90°-95° C. was found to be ideal.    -   Low pressure, high volume air delivered to a mixing chamber just        prior to delivery of the hot solution produced the best results.        Although compressed air may be used, the disadvantages include        the higher cost of the compressor and increased cooling of the        foam as the air expands.    -   Providing negligible outlet pressure was also found to be        important and a restrictive nozzle at the outlet should be        avoided.    -   Aerating the hot foamable solution at the treatment lance, such        that the foam flows through only a short delivery passage was        found to provide the best, most consistent and economic results.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof.

1. A method of controlling vegetation, including the steps: preparing a foam having a temperature of between 75° C. and 100° C. and consisting volumetrically of 60% to 75% aqueous solution of biodegradable foaming agent and 40% to 25% air, and directing a stream of the foam against the surface to be treated.
 2. A method of controlling vegetation as claimed in claim 1 wherein the foam is prepared by heating the aqueous solution and mixing a stream of air with a stream of aqueous solution to produce the stream of foam which is discharged at low pressure.
 3. A method of controlling vegetation as claimed in claim 1 wherein the aqueous solution is alkyl polygclycoside mixed with water at a dilution rate of approximately 0.4%.
 4. A method of controlling vegetation as claimed in claim 1, wherein the foam is produced by: (a) providing a flow of hot aqueous solution of biodegradable foaming agent; (b) passing at least part of said flow through a channel to facilitate low pressure that creates a venturi to draw in a stream of ambient air; (c) mixing the stream of air with the flow of hot aqueous solution to produce the stream of foam.
 5. A method of controlling vegetation as claimed in claim 4 wherein the channel produces a free jet of solution.
 6. A method of controlling vegetation as claimed in claim 1, wherein the solution is pumped through a boiler, the full stream of solution flowing through one channel to facilitate low pressure that creates a venturi to draw in the stream of ambient air into a mixing chamber, where it is mixed with the solution to produce foam that then passes through an elongate delivery passage before being discharged through a divergent shroud.
 7. An apparatus for performing the method of controlling vegetation substantially as described above, the apparatus including: a) means for supplying a stream of hot aqueous solution; b) means for supplying a stream of air, c) a mixing chamber where the streams of air and solution are mixed to form foam, and d) an elongate delivery passage connected between the mixing chamber and a divergent outlet shroud.
 8. The apparatus of claim 7 wherein the means for supplying a stream of air includes channel means which facilitate low pressure that creates a venturi to draw in a stream of ambient air.
 9. The apparatus of claim 7 for controlling vegetation by the application of hot foam through a hand lance, including a hand-operated valve for controlling the flow of solution into the mixing chamber and a nozzle mounted on the hand lance for producing the venturi.
 10. (Canceled)
 11. (Canceled)
 12. A method of controlling vegetation as claimed in claim 2 wherein the aqueous solution is alkyl polygclycoside mixed with water at a dilution rate of approximately 0.4%.
 13. A method of controlling vegetation as claimed in claim 2, wherein the foam is produced by: (a) providing a flow of hot aqueous solution of biodegradable foaming agent; (b) passing at least part of said flow through a channel to facilitate low pressure that creates a venturi to draw in a stream of ambient air; (c) mixing the stream of air with the flow of hot aqueous solution to produce the stream of foam.
 14. A method of controlling vegetation as claimed in claim 3, wherein the foam is produced by: (a) providing a flow of hot aqueous solution of biodegradable foaming agent; (b) passing at least part of said flow through a channel to facilitate low pressure that creates a venturi to draw in a stream of ambient air; (c) mixing the stream of air with the flow of hot aqueous solution to produce the stream of foam.
 15. A method of controlling vegetation as claimed in claim 2 wherein the solution is pumped through a boiler, the full stream of solution flowing through one channel to facilitate low pressure that creates a venturi to draw in the stream of ambient air into a mixing chamber, where it is mixed with the solution to produce foam that then passes through an elongate delivery passage before being discharged through a divergent shroud.
 16. A method of controlling vegetation as claimed in claim 3 wherein the solution is pumped through a boiler, the full stream of solution flowing through one channel to facilitate low pressure that creates a venturi to draw in the stream of ambient air into a mixing chamber, where it is mixed with the solution to produce foam that then passes through an elongate delivery passage before being discharged through a divergent shroud.
 17. A method of controlling vegetation as claimed in claim 4 wherein the solution is pumped through a boiler, the full stream of solution flowing through one channel to facilitate low pressure that creates a venturi to draw in the stream of ambient air into a mixing chamber, where it is mixed with the solution to produce foam that then passes through an elongate delivery passage before being discharged through a divergent shroud.
 18. A method of controlling vegetation as claimed in claim 5 wherein the solution is pumped through a boiler, the full stream of solution flowing through one channel to facilitate low pressure that creates a venturi to draw in the stream of ambient air into a mixing chamber, where it is mixed with the solution to produce foam that then passes through an elongate delivery passage before being discharged through a divergent shroud.
 19. The apparatus of claim 8 for controlling vegetation by the application of hot foam through a hand lance, including a hand-operated valve for controlling the flow of solution into the mixing chamber and a nozzle mounted on the hand lance for producing the venturi. 